The invention relates to a PTC temperature sensor of the type which includes a PTC resistor connected to conductor webs disposed in a housing and also a process for producing PTC temperature sensing elements for the PTC temperature sensor.
It is generally known to use temperature sensors with temperature sensing elements containing thermally resistant resistor materials having temperature-dependent resistance values for measuring comparatively high temperatures such as prevail, for example, in exhaust gases from internal combustion engines (ef. E. D. Macklen, "Thermistors", published by Electrochemical Publications Ltd., 1979).
PTC temperature sensors utilize the continuous change in the resistance of metals or semiconductors having positive temperature coefficients with varying temperatures. The metals preferably used in PTC temperature sensors are platinum and nickel because of their high stability and reproducibility.
It is furthermore known, for example from EP-A-0,18,900 and 0,142,993 and also German Offenlengungsschrift 3,017,947 and 3,543,759, to use planar exhaust gas sensors, which can be prepared in a particularly cheap manner by ceramic film technology and screen printing technology, to determine the .lambda.-value of gas mixtures. Disadvantages of the known PTC temperature sensors are the fact that they age too rapidly during use, that their response times are too long, their method of production is too expensive and/or their dimensions are too large. The unavoidable diffusion of compounds of the gas to be measured e.g. hydrogen through highly refractory metal housings or through glass envelopes, in particular, causes undesirable changes i the resistance values. Metallic PTC resistors are put at risk at high temperatures also by oxidation in particular.